The correct option is B If both assertion and reason are true but reason is not the correct explanation of the assertion.
→A.→B=|→A||→B| cosθ=0
→A×→B=|→A||→B| sinθ=0
If →A and →B are not null vectors, then both sinθ and cosθ cannot be zero simultaneously, so it is essential that one of the vector must be null vector.